A supercapacitor can be broadly classified as an electric double layer capacitor (EDLC) which employs carbon particles or fibers having a high specific surface area as an electrode material or as a pseudocapacitor comprising a metal oxide or a conductive polymer.
An EDLC exhibits a long lifetime but due to the accumulation of charges only on the surface of the electric double layer, the capacitance thereof is lower than that of a metal oxide-based or an electric conductive polymer-based supercapacitor.
The metal of a metal oxide-based supercapacitor is capable of undergoing a change in its multiple valence states, which allows oxidation and reduction reactions to take place. Such oxidation and reduction reactions corresponding to charging and discharging processes require the ion and electron to move rapidly between the electrolyte and the electrode, and accordingly, the electrode is preferred to have a high specific surface area, and the electrode active material, a high electric conductivity.
Since ruthenium oxide heat-treated at a low temperature exhibits a very high specific capacitance as well as a high electric conductivity similar to that of a metal (J. Electrochem. Soc. 142, 2699 (1995)), a number of studies have been conducted to use ruthenium oxide as an electrode material for a supercapacitor. However, ruthenium is expensive and only the ruthenium oxide exposed on the surface of the ruthenium oxide particles participates in the oxidation-reduction reaction. Therefore, a variety of methods such as forming a complex of ruthenium oxide with an inexpensive metal oxide or a carbon material, or depositing ruthenium oxide on a support having a large specific surface area have been attempted.
Various carbon materials have been considered for use as a support. Active carbon has a high specific surface area of 1,000 m2/g or more, but has a low electric conductivity; while carbon black has an excellent electric conductivity, but the specific surface area thereof is very low, about 10 m2/g or less. Therefore, an electrode for a supercapacitor prepared by depositing a metal oxide thin layer on such carbon materials has a tendency to exhibit a high specific capacitance at low-speed charging and discharging cycles, but a low specific capacitance at a high-speed charging and discharging. A supercapacitor employed in fuel cell-driven or hybrid automobiles is required to have good high-speed charging and discharging capabilities.
Accordingly, there has been a need to develop an electrode for a supercapacitor having a high specific surface area and a good electric conductivity which is capable of maintaining a high specific capacitance during high-speed charging and discharging cycles.